Recently, energy saving in automobiles and construction or agricultural machinery, i.e., fuel saving has become an urgent need in order to deal with environmental, issues such as reduction in carbon dioxide emissions, and units such as engines, transmissions, final reduction gears, compressors, or hydraulic power units have been strongly demanded to contribute to energy saving. Consequently, the lubricating oils used in these units are required to be reduced in stir resistance and frictional resistance more than before.
Reduction of the viscosity of a lubricating oil is exemplified as an effective energy saving means. For example, an automobile automatic transmission or continuously variable transmission has a torque converter, a wet clutch, a gear bearing mechanism, an oil pump and a hydraulic control system while a manual transmission or final reduction gear unit has a gear bearing mechanism. Reduction of the viscosity of a lubricating oil to be used in such transmissions can reduce the stir and frictional resistances in the gear bearing mechanism and oil pump and thus enhance the power transmission efficiency, resulting in an improvement in the fuel efficiency of an automobile.
However, reduction of the viscosity of the lubricating oil used in these transmissions and units may cause the above-described units and mechanisms thereof to be significantly shortened in fatigue life or reduced in extreme pressure properties and may generate seizure, possibly resulting in some malfunctions in the transmissions or final reduction gear units.
Examples of conventional automobile transmission oils which enable a transmission to maintain various properties thereof such as shifting properties for a long time include those produced by optimizing and blending synthetic and/or mineral base oils, antiwear agents, extreme pressure additives, metafile detergents, ashless dispersants, friction modifiers and viscosity index improvers (for example, see Patent Literature Nos. 1 to 3 below).
However, since these compositions are all are not intended to improve fuel, economy, they are high in kinematic viscosity and have not been sufficiently studied with regard to effects on extreme pressure properties or metal-to-metal friction when lowered in viscosity.